Electron Shell

Summary: An energy level around the nucleus where electrons orbit. The first shell holds up to 2 electrons; subsequent shells hold up to 8 (for the first 20 elements). Tags: igcse chemistry definition atomic-structure Created: 2026-07-14 Last Updated: 2026-07-14

An electron shell, also known as an energy level, is a region at a fixed distance from the nucleus in which electrons with a specific range of energies are most likely to be found. According to the Bohr model used at IGCSE level, electrons occupy discrete shells labelled n = 1, 2, 3, and so on, with the first shell (closest to the nucleus) having the lowest energy and holding a maximum of 2 electrons, while the second and third shells each hold a maximum of 8 electrons for the first 20 elements. Electrons fill the lowest available energy shell first — a rule known as the Aufbau principle — which dictates the electronic configuration of every atom and explains the periodicity of chemical properties across the Periodic Table. The number of electrons in the outermost shell (the valence shell) equals the group number for Groups 1 to 7 and determines how the element will bond and react; atoms with a full outer shell, such as the noble gases in Group 8/0, are chemically unreactive. Exam questions routinely test the ability to draw and interpret shell diagrams (using the 2,8,8 notation) for elements up to calcium (atomic number 20) and to relate outer-shell electron count to group number and chemical behaviour.

Key term — IGCSE Chemistry (0620/0971) glossary.